Conventional ink-jet recording devices have jet-openings, or orifices, arranged to correspond to the desired pel density. Such devices have ink contained in a chamber communicating with the openings or orifices, and employ pressure pulses which are selectively applied to jet the ink from the corresponding jet-openings onto paper.
In apparatus of this type, it is necessary to set the volume ratio of each orifice at a relatively high value in order to maintain the ink jetting capability of the orifice. Thus, it is difficult to miniaturize the ink jetting devices. Also, it is necessary to set the pitch, or interval between adjacent jet-openings, at a relatively high value, so that the image recording density cannot be high. Further, recording speed is lowered because mechanical scanning is employed to apply the pressure pulses used to jet the ink.
In order to solve such problems, various structures, such as the following, have been proposed. A first system, the magnetic ink-jet system, provides magnetic ink in the vicinity of an array of magnetic electrodes, and ink jetting is accomplished at a corresponding image density by use of a magnetic field applied to the meniscus of the ink. The magnetic ink is then jetted toward a recording sheet by use of a patterned electric field formed between the electrode array and an electrode disposed opposite the array on the other side of a recording sheet (Japanese Patent Unexamined Publication No. 37163/1981).
In addition to the aforesaid methods, the so-called plane ink jet method is also well known. This method, involves arranging ink in a slitlike inkholder in parallel to an electrode array, and jetting the ink in accordance with an electric field pattern formed between an electrode facing the electrode array so that ink droplets selectively impact upon an intervening recording paper. Since no minute orifice for storing ink is required in this method, ink clogging can be prevented. However, a high voltage must be applied to jet the ink droplets which makes it necessary to drive the electrode array on a time division basis to prevent a voltage leak across the adjoining or neighboring electrodes. Consequently, the recording speed is limited.
Another system, the thermal bubble-jet system, uses apparatus wherein heat is rapidly applied to the ink to cause film-surface boiling. The ink is then jetted from an orifice by the pressure increase caused by the rapid growth of bubbles within the orifice (Japanese Patent Unexamined Publication No. 1611664/1980).
In the ink-jet recording systems described above, high-density recording can be made, and because electrical scanning can be carried out, high-speed recording can be achieved in most systems. However, in the magnetic ink-jet system, it is necessary to use ink containing magnetic powder which causes the ink to be black and makes it difficult to reproduce a color picture by superimposing several colors of ink to form an image. The plane scanning ink-jet system is disadvantageous in that it is necessary to apply a high voltage to jet the ink. Therefore, time division driving of the electrode array is required to prevent voltage leakage between adjacent electrodes. This is unsuitable for high-speed recording. Furthermore, in the apparatus using the thermal bubble-jet system, it is necessary to apply heat rapidly to cause film-surface boiling, resulting in thermal deterioration of the protective layer applied to the resistance heaters used to heat the ink.
The present invention provides a thermal electrostatic ink-jet recording apparatus in which thermal energy, corresponding to image information, is selectively applied to the recording ink in an electric field and the heated ink portions are jetted toward a recording sheet by use of a predetermined electrostatic field.
In apparatus of this latter type, magnetic ink used in the magnetic ink-jet system is not required. Accordingly, not only can color imaging be easily attained by superimposing several kinds of ink during printing, but also voltage leakage in the vicinity of the electrode array can be effectively prevented because the extremely high intensity electric field required in the plane ink-jet system (in which ink is jetted only by an electrostatic field) is not required for jetting ink. Furthermore, the quantity of thermal energy can be reduced because the very large thermal energy required in the bubble-jet method (in which ink is jetted only by thermal energy) is not required for jetting ink. Accordingly, thermal deterioration of ink can be effectively prevented, and high-speed, high-density recording can be accomplished while effectively eliminating the above defects in conventional apparatuses.
In such a thermal electrostatic ink-jet recording apparatus, an exothermic array, composed of a plurality of electrical resistance heating elements that are disposed at intervals corresponding to pel density, is used as means for applying thermal energy to the ink. The array of heaters is provided at an edge portion of the ink-jet recording head in the vicinity of an orifice in the ink reservoir located in the head body, so that ink within the ink reservoir is indirectly heated. In this case, an insulating substrate constituting one side wall of the ink reservoir is subjected to a photolithographic process or the like to form the array of heating elements.
While the thermal electrostatic ink-jet printer above has been found to be very effective, it is difficult in producing the exothermic array to arrange the heating elements in the vicinity of the orifice side end portion of the insulating substrate while maintaining the array in a plane. Generally, it is required that the exothermic array be arranged a certain distance apart from the the edge portion of the insulating substrate, and then the end portion of the substrate is cut so as to properly position the exothermic array relative to the edge portion of the insulating substrate. Further, in order to keep recording image quality good it is necessary to stabilize the jetting of the ink. Formation of a stable ink meniscus is one of the means for stabilizing the jetting of the ink. In the conventional apparatus, however, it happens that the surface of the end portion is rough because the end portion of the head body is cut, as described above. Therefore, it is necessary to polish the surface of the end portion to make the surface satisfactorily smooth. In short, a problem exists in that the process of producing the head body is troublesome and sometimes not effective.
Furthermore, since the exothermic array is attached or fixed onto the insulating substrate, the heat from the heating resistors naturally escapes toward the insulating substrate, resulting in a problem that the efficiency of heat transmission to the ink becomes correspondingly poor.